The McArthur river in Borroloola in the Northern Territory. A zinc-lead mine on the river caused combustion and sulphur-dioxide emissions when it dumped waste in the wrong place.
Photograph: Helen Davidson for the Guardian

An Australian mine owned by the global trading firm Glencore mistakenly dumped 63 truckloads of dangerous waste material in the wrong place, where it combusted and sent sulphur dioxide into the atmosphere.

The scale of the incident, which occurred at the remote McArthur river zinc-lead mine in Australia’s north, was kept out of the public eye. The Northern Territory government ordered an investigation but refuses to release any details, claiming no report exists because the findings were delivered verbally.

The 63 truckloads of reactive rock – known as potentially acid-forming (PAF) rock – were never removed. Observers have expressed concern that even with the remedial work since carried out by the company, the approaching monsoons could cause another chemical reaction.

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The only public admission by the mine operator, McArthur River Mining (MRM), is one paragraph within a draft environmental impact statement running to several thousand pages.

The waste rock, estimated at about 14,000 tonnes, was mistaken for more benign material, the company admitted, and dumped on the company’s less secure southern waste facility. It should have gone to the company’s northern waste dump, which has better containment features.

The company’s impact statement says “the presence of sulphur dioxide and surface cracking in areas of the [southern waste dump] in 2016 were detected and an investigation undertaken. It was found that 63 truckloads of PAF ... was incorrectly mined as [moderate risk rock] and placed on the [southern dump]”.

In a series of contradicting statements and interviews, the NT Department of Primary Industry and Resources confirmed it had hired an independent investigator but that the incident prompted no sanctions or prosecutions because the way MRM’s authorised mine management plan was written “didn’t exclude them from putting material there”.

The possible environmental damage of the PAF is significant and could include contamination of the river system.

The trucks, similar to this one, carry more than 200 tonnes in each load. 63 truckloads of reactive PAF were taken to the southern facility. Photograph: David Gray/Reuters

According to the mine’s independent monitor, reactive PAF has a higher risk of spontaneously combusting if not managed properly and is “likely to generate excessive heat and produce toxic gases” if it does. It has the highest capacity to generate acidic and metalliferous drainage, and must be prevented from being in contact with water and oxygen.

“If there are significant chemical reactions in the [southern facility], this will lead to mobilisation of acid, salts and heavy metals into any drainage from this waste,” said Gavin Mudd, environmental engineer at Monash University and chair of the Mineral Policy Institute.

“If water management systems fail, this could potentially lead to acidic, saline and heavy metal-rich waters entering the McArthur river – leading to significant risks for biodiversity and humans.”

MRM said in its draft EIS that the PAF rock represented 0.24% of the total mass of the southern waste rock facility, and had “a negligible impact on the overall acid balance” because of the far greater “acid consuming capacity” within the pile.

But in 2016 sulphur dioxide emissions were detected at the site and in early August the then mines minister, David Tollner, announced staff from his department were investigating.

The sitting Northern Territory government was then defeated by Labor in the August 2016 election and no further information about the investigation has ever been released.

The department refused to provide its findings, telling Guardian Australia the investigation was for “internal purposes”. However, it did say that McArthur River Mining had “operated in accordance” with its mine management plan.

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It said MRM had undertaken “interim remediation” activities to compact and cap the material – extracting the oxygen and protecting it from rain – and there had been no further venting. It later said the remediation measures could be interim or permanent, because future management of the waste was now covered by the operator’s draft environmental impact statement (EIS).

MRM is seeking to expand its operations beyond the currently scheduled end date of 2018. Its EIS – which contained the dumping admission – is under public assessment and a government decision is expected imminently.

The NT department’s head of mines, Armando Padovan, said its mining authorisation documents – which cover the mine management plans – were not strong enough to allow prosecution at the time.

He said the authorisations had been strengthened since the incident, and suggested prosecution would be an option for the department if the same thing were to happen again.

Padovan questioned whether the estimated 14,000 tonnes was a significant amount in the context of “a mine of that magnitude”.

“I think if you compare it to a small extractive operation it’s a lot of material, but we’ve got a major mine here.”

Various explanations of the investigator’s findings were given.

The director of mining compliance for the NT government, Roslyn Vulcano, first said there was no report, then said there was one finding that was “internal documentation”, but said she had no paperwork about the investigation on hand. She would not say if she had seen any.

In a subsequent interview requested by the department, Padovan said the findings were delivered verbally.

“This happened over a year ago and a lot has happened since then,” he said. “What the focus turned to was how to deal with the material, what was the best way of moving forward.”

In explaining the lack of a report, Padovan said: “We wanted to quickly move on and deal with how do we address the issue of the material that’s been placed there. It’s really about allocating resources, and where the risk is, and how we manage the risk.”

Department staff also gave contradictory statements describing the investigation variously as internal, independent, and conducted by McArthur River Mining, and Padovan also questioned the legitimacy of the inquiry as “a formal investigation”.

Padovan instead suggested that the fact there was no report defined the investigation as informal, even though it was conducted by a contracted independent investigator.

The reactive PAF mistake was at least the second such incident involving MRM. In 2015, the operator was found to have dumped moderate-risk waste rock in the southern facility “using techniques no longer considered best practice”.

In that incident the department found McArthur River Mining had performed an act of “non-compliance” and ordered the operator to remove the waste rock, which should have been in another facility. It did not.

Instead the company opted to appeal before the mining advisory committee, which found in favour of its argument that the southern facility was a “temporary facility necessitated by a restriction” on dumping volatile rock on the northern pile, which was being redesigned after the 2014 reclassification found it contained almost 90% problematic rock, not the 16% previously thought.

Its decision allowed MRM to “maintain all current material” at the southern waste rock facility until the completion of its environmental impact assessment.

The advisory committee has been criticised in the past – in particular at the time of that decision – for being stacked with representatives from the mining industry.

The McArthur river mine has been extraordinarily divisive in the community because of repeated alleged environmental contamination on one hand, and its provision of jobs and other support to the local economy on the other.

The NT is the only Australian jurisdiction to measure mining royalties based on profit. Glencore has paid no royalties from the McArthur river mine for the past two years at least, because, it said, the system allowed it to make capital investment offsets.

The minister for primary industry and resources, Ken Vowles, declined requests for interview, and did not answer written questions. In a statement he said the incident happened under the former government and at the time the department confirmed MRM had acted within their management plan.

“The MMP has since been amended. [The department] will continue to monitor and regulate the MRM site, as they do all mining sites in the territory.”